1. Introduction
This invention relates to a nickel electroplating solution, compounds useful in the electroplating solution, and processes for use of the electroplating solution.
2. Description of the Prior Art
Electrodeposition processes of nickel and other compounds often provide a dull or discolored substrate surface in low current density areas. This problem has been addressed with varying success through addition of brightening and leveling agents to an electroplating solution.
Specifically, several acetylenic alcohols and diols have been used as brightening agents in nickel electroplating solutions. See, for example, U.S. Pat. No. 3,711,384; and F. A. Lowenheim, Modern Electroplating, pp. 271-76 (2d ed., John Wiley & Sons). Nitrogen-containing ring compounds have been used as levelers and certain pyridinium compounds have been used in combination with certain acetylenic compounds. See, for example, U.S. Pat. Nos. 2,876,177 and 3,862,019, both incorporated herein by reference. Brightening systems such as these can be crucial to achieve an acceptable nickel deposit and industry would clearly benefit by the discovery of additional brightening agents.
Nickel plating of a surface having irregular topography poses particular difficulties. During electroplating a voltage drop exists along the irregular surface resulting in an uneven nickel deposit. Where the voltage drop is extreme, that is, where the surface irregularity is substantial, it may be not possible to satisfactorily plate the surface.
Thus, successful metal plating is frequently a challenging step in the manufacture of printed circuit boards. Printed circuit boards often have "through-holes", perforations through the board surface to provide attachment means for the board hardware and, in the case of a multilayer board, to provide interconnection between each board layer. Processes for formation of conductive through-holes are well known and described in numerous publications including U.S. Pat. No. 4,515,829, incorporated herein by reference. The walls of a through-hole are metalized to provide conductivity between the multiple circuit layers of the board. Electroless plating procedures are used to form a first metallic coating over the through-hole wall and then an electrolytic deposit is employed to enhance the electroless layer. Nickel is often pattern plated over a copper deposit to provide a barrier layer which prevents diffusion between the underlying copper layer and a subsequently applied metal layer. For example, gold is frequently plated over such a nickel barrier layer to provide a metal etch resist. See, generally, Coombs, Printed Circuit Handbook, p. 7-22 (2d ed. 1979).
Manufacture of an acceptable printed circuit board having through-holes requires electroplating completely through the length of the barrel of the hole from the surface pad on each board side. A surface pad is a plated area on the plane surface of a printed circuit board through which a through-hole is drilled.
A voltage drop exists between the surface pad and the midpoint of the barrel of a through-hole. This voltage drop is a function of several factors including the through-hole's aspect ratio. The term "aspect ratio" refers to the thickness of a printed circuit board divided by the diameter of the through-hole of the board.
The thickness of electrodeposited metal is usually at a maximum at the plane surface of the surface pad tapering to a minimum midway along the length of the through-hole. With prior nickel electroplating systems, satisfactory nickel plating of high aspect ratio through-holes has been difficult or simply not possible. Nickel will be either completely absent or plated too thin midway along the length of the through-hole walls. Such inadequate plating results in circuit defects and board rejection.
Thus, to satisfactorily plate a through-hole, a plating solution must have adequate throwing power. In the case of a printed circuit board having through-holes, the term "throwing power" is defined as the ratio of thickness of metal deposited in the mid-barrel of a through-hole to the thickness of metal plated on the through-hole's surface pad. Additionally, it should be clear that throwing power is a function of the aspect ratio of the through-hole being plated. For example, a plating solution may exhibit a throwing power of 1:1 for a low aspect ratio board, but when used to plate a high aspect ratio board the thickness of metal deposited at the through-hole mid-barrel may only be a fraction of a mil (or zero) for every mil of metal plated at the surface pad.
In B. F. Rothschild, Electronic Packaging and Production, vol. 15, p. 102 (Aug. 1975), throwing power of an acid copper plating solution was reported to be enhanced through increasing the ratio of sulfuric acid concentration to copper ion concentration. For instance, high throw acid copper baths have been employed with an acid to copper ion ratio of ten to one. See, L. Mayer, et al., Plating and Surface Finishing. pp. 46-49 (March 1981). Nickel plating solutions, however, are typically buffered with boric acid and the acid to metal ion ratio can not be well-controlled as in copper systems. The throwing power of a nickel plating solution has been enhanced by increasing the bath's conductivity, for example through use of an all-nickel chloride bath. However, this approach imparts only relatively limited throwing power and consequently does not enable satisfactory nickel plating of printed circuit boards having through-holes of high aspect ratios. Further, an all-nickel chloride solution provides a nickel deposit of relatively high internal stress which may be undesirable in many applications.